1. Related Field
This invention relates to a method for production of a three-dimensional body by successively providing powder layers and fusing together of selected areas of said layers, which areas correspond to successive cross sections of the three-dimensional body.
2. Description of Related Art
Equipment for producing a three-dimensional object layer by layer using a powdery material which can be fused together and solidified by irradiating it with a high-energy beam of electromagnetic radiation or electrons are known from e.g. U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,647,931 and SE524467. Such equipment include for instance a supply of powder, means for successively applying layers of powder on a vertically adjustable platform or working area, and means for directing the beam over the working area. The powder sinters or melts and solidifies as the beam, layer by layer, moves over the working area.
When melting or sintering a powder using a high-energy beam, it is important to have a thorough control of the temperature of the irradiated material to provide the object with appropriate material properties and to avoid geometrical deformations. For instance, a too high local temperature might destroy the object being produced and a too inhomogeneous temperature distribution might lead to cracks. Further, to provide for a thorough fusion the temperature of the upper layers of the powder bed should normally be kept above a minimum value during the melting step. Besides keeping control of the temperature it is normally important to try to reduce the production time, i.e. to try to sweep the beam as efficiently as possible over the selected area.
Only a selected part or area of each powder layer is fused together. The beam sweeps in a certain path over each selected area in a scan or hatch pattern that makes the area completely fused together. Often, this scan pattern has the form of parallel lines distributed at equal distances over the selected area. Each of these selected areas, which may include several part areas, corresponds to a cross section of the object being built up in the powder bed.
Sweeping the beam in a scan pattern with parallel lines can be done by scanning the lines in order. Due to heat transfer from heated material along previously scanned lines, the temperature in the material along a certain line to be scanned will be higher than the starting temperature (i.e. higher than the temperature in the material when the first line is scanned). At least when using a high-energy beam this temperature build-up must be taken into account in order to maintain an appropriate local temperature within the material.
One way of taking this into account is to adjust the beam energy input in response to the temperature build up. This could, for instance, be done by varying the beam power or by varying the speed at which the beam moves over the powder layer. An example is to increase the beam speed at beam turning positions where the end of a first scan line is close to the beginning of a second scan line. However, to do this properly it is needed to have information on the temperature in the material. This temperature, or more exactly the surface temperature of the powder bed, can be measured using a heat camera. Real-time corrections or controlling of the beam based on input from such a camera is, however, difficult to perform properly because of the long response time of the system (even if actions are taken to decrease the temperature immediately when an increased temperature has been detected the temperature is likely to continue increasing for some time). A heat camera may yet be useful for checking, after the production, whether anything went wrong in the production process.
U.S. Pat. No. 5,904,890 discloses a method where the beam scan speed is varied as a function of length of the scan lines in a scan pattern with parallel lines. The beam speed is lower for longer scan lines and higher for shorter lines as to avoid varying cooling when the beam is away from a certain area. The purpose is to achieve a homogeneous density distribution in the product produced. This method may be useful with regard to the above-mentioned temperature build-up if the beam speed is high compared with the length of the scan lines. However, if the scan lines are long the beam speed should be adjusted only at the end parts of the scan lines, and if the lines are distributed over several selected areas of the same powder layer or in a different pattern the temperature build up will not be similar at all parts of the area(s). Moreover, if the beam energy is high a more complex scan pattern may be required. In such cases the temperature build-up will not be properly taken into account just by varying the beam speed with respect to the length of the scan lines.
WO 2008/013483 discloses a method where parallel scan lines are scanned in a particular order so that a minimum security distance is established between consecutively scanned lines. Temperature (and charged particle) build-up between the scan lines is thus taken into account by preventing the occurrence of heat transfer interference between consecutively scanned lines. The method is primarily intended for pre-heating of the powder layer with a high beam speed and high beam power but could also be used for avoiding heat transfer interference during the step of melting the powder. However, this would lead to a rather time-consuming production process.
Thus, there is need for more elaborated scanning strategies which allows for a thorough temperature control as well as a time-efficient production.